1. Field of the Invention
This invention relates to growing thin films on silicon carbide substrates.
2. Background of the Invention
Metal-oxide-semiconductor are structures which include a silicon-carbide(SiC) substrate and an oxide film grown on the silicon-carbide (SiC) substrate. MOS diodes, MOSFETs and MOSICs are formed on a silicon carbide substrate and used in the art.
Silicon Carbide (SiC) takes various kinds of crystal structures (referred to as polytype), and has a band gap between 2.4 eV and 3.3 eV depending on the crystal structure. The silicon carbide is thermally, chemically and physically stable, and resistant to high-energy particle radiation. Accordingly, silicon carbide (SiC) is favorable for the semiconductor material of, for example, a high temperature operating device, a high power device, a high reliability semiconductor device and a radiation resistant device. The silicon carbide (SiC) is also known to be free from certain problems related to pollution and resources.
It is difficult to effectively form an oxide layer on a SiC semiconductor. Where conventional silicon material thermal oxidation methods are employed to form the oxide layer, the oxidation rate is very slow, and a sharp boundary is not formed between the oxide layer and the SiC substrate.
Thin films of silicon dioxide are conventionally grown on silicon carbide (SiC) substrates at atmospheric pressure in a chemical ambient containing water vapor or other oxidant species at temperatures in the range of 1,000° C. to 1200° C. The film growth rate is slower than that for single crystal silicon by at least a factor of 10. In addition, following the oxidation process an additional many time consuming (3–4 hour) wet ambient process is performed at 950° C. (considered below the temperature at which appreciable oxide would form on SiC) to improve the interface quality between the oxide and the silicon carbide substrate. The process produces carbonaceous clusters up to tens of nanometers in height that are formed as a result of the thermal oxidation of the silicon carbide single crystal.
Atmospheric oxidation in dry O2 requires 14 hours at 1,050° C. to grow 200 Å of film on SiC and 4 hours in wet ambient at the same temperature produces 200 Å.
At atmosphere and aggressive pyrogenic steam ambient, steam formed by reacting O2 with H2 at T>700° C., SiC oxidizes slowly and only at temperatures above roughly 1,000° C. Under comparable conditions single crystal silicon oxidizes at a rate at least ten times faster. Forming silicon containing dielectric films such as SiO2 on a substrate such as SiC at a lower temperature and with less time will be beneficial.
In summary, silicon carbide oxides are slow to form and require temperatures above 1,000° C.
What is needed, then, is an improved method for effectively forming an oxide layer on a SiC semiconductor. The improved method should yield a faster oxidation rate and a sharp boundary between the oxide layer and the SiC substrate.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in this art how the identified needs could be met.